The present invention relates to electrical nerve stimulation, and more particularly, electrostimulation of the nerve based on channel specific sampling sequences.
Cochlear implants (inner ear prostheses) are a possibility to help profoundly deaf or severely hearing impaired persons. Unlike conventional hearing aids, which just apply an amplified and modified sound signal, a cochlear implant is based on direct electrical stimulation of the acoustic nerve. The intention of a cochlear implant is to stimulate nervous structures in the inner ear electrically in such a way that hearing impressions most similar to normal hearing are obtained.
A cochlear prosthesis essentially consists of two parts, the speech processor and the implanted stimulator. The speech processor contains the power supply (batteries) of the overall system and is used to perform signal processing of the acoustic signal to extract the stimulation parameters. The stimulator generates the stimulation patterns and conducts them to the nervous tissue by means of an electrode array which usually is positioned in the scala tympani in the inner ear. The connection between speech processor and stimulator is established either by means of a radio frequency link (transcutaneous) or by means of a plug in the skin (percutaneous).
At present, the most successful stimulation strategy is the so called xe2x80x9ccontinuous-interleaved-sampling strategyxe2x80x9d (CIS), as described by Wilson B. S., Finley C. C., Lawson D. T., Wolford R. D., Eddington D. K., Rabinowitz W. M., xe2x80x9cBetter speech recognition with cochlear implants,xe2x80x9d Nature, vol. 352, 236-238. (July 1991) [hereinafter Wilson et al., 1991], which is incorporated herein by reference. Signal processing for CIS in the speech processor involves the following steps:
(1) splitting up of the audio frequency range into spectral bands by means of a filter bank,
(2) envelope detection of each filter output signal,
(3) instantaneous nonlinear compression of the envelope signal (map law).
According to the tonotopic organization of the cochlea, each stimulation electrode in the scala tympani is associated with a band pass filter of the external filter bank. For stimulation, symmetrical biphasic current pulses are applied. The amplitudes of the stimulation pulses are directly obtained from the compressed envelope signals (step (3) of above). These signals are sampled sequentially, and the stimulation pulses are applied in a strictly non-overlapping sequence. Thus, as a typical CIS-feature, only one stimulation channel is active at one time. The overall stimulation rate is comparatively high. For example, assuming an overall stimulation rate of 18 kpps, and using an 12 channel filter bank, the stimulation rate per channel is 1.5 kpps. Such a stimulation rate per channel usually is sufficient for adequate temporal representation of the envelope signal.
The maximum overall stimulation rate is limited by the minimum phase duration per pulse. The phase duration cannot be chosen arbitrarily short, because the shorter the pulses, the higher the current amplitudes have to be to elicit action potentials in neurons, and current amplitudes are limited for various practical reasons. For an overall stimulation rate of 18 kpps, the phase duration is 27 xcexcs, which is at the lower limit.
Each output of the CIS band pass filters can roughly be regarded as a sinusoid at the center frequency of the band pass filter, which is modulated by the envelope signal. This is due to the quality factor Q≈3 of the filters. In case of a voiced speech segment, this envelope is approximately periodic, and the repetition rate is equal to the pitch frequency.
In the current CIS-strategy, the envelope signals only are used for further processing. i.e., they contain the entire stimulation information. For each channel, the envelope is represented as a sequence of biphasic pulses at constant repetition rate. As a characteristic feature of CIS, this repetition rate (typically 1.5 kpps) is equal for all channels, and there is no relation to the center frequencies of the individual channels. It is intended that the repetition rate is not a temporal cue for the patient, i.e., it should be sufficiently high, so that the patient does not percept tones with a frequency equal to the repetition rate. The repetition rate is usually chosen greater than at twice the bandwidth of the envelope signals (Nyquist theorem).
In accordance with one aspect of the invention, electrodes in a multichannel electrode array are activated using channel specific sampling sequences. A channel specific sampling sequence for each electrode is defined, having a particular duration, amplitude, and number of pulses. A weighting factor is applied to the channel specific sampling sequence, creating a weighted channel specific sampling sequence. Each electrode in the multichannel electrode array is then simultaneously activated using sign-correlated pulses, the sign-correlated pulses based on parameters of spatial channel interaction, non-linear compression, and each electrode""s weighted channel specific sampling sequence.
In accordance with other related embodiments, the electrodes stimulate the acoustic nerve. The multichannel electrode array can be used in a monopolar electrode configuration having a remote ground. The pulse amplitudes can be derived by sampling a signal waveform, for example, one half the period of a sinusoid between 0 and xcfx80, or one quarter of a sinusoid between 0 and xcfx80/2 so that the amplitude distribution is monotonically increasing. Symmetrical biphasic current pulses can be used to sample the signal waveform. The channel specific sampling sequence pulse rate may be between 5-10 kpps. The parameters of spatial channel interaction can be based on a single electrode model having exponential decays of the potentials at both sides of the electrode, the sign-correlated pulses having amplitudes which are calculated using properties of a tri-diagonal matrix. The multichannel electrode array can be in a cochlear implant, whereby the weighting factor is transmitted to the cochlear implant. Start and stop bits, and addresses associated with an electrode can also be transmitted to the cochlear implant.
In accordance with another embodiment of the invention, electrodes in a multichannel electrode array are activated using channel specific sampling sequences by applying an acoustic signal to a bank of filters, each filter in the bank of filters associated with a channel having an electrode. A weighting factor is derived for each channel based on the output of each channel""s filter. The weighting, factor is then applied to a channel specific sampling sequence having a particular duration, amplitude and number of pulses, creating a weighted channel specific sampling sequence. Each channel""s electrode is simultaneously activated using sign-correlated pulses, the sign-correlated pulses based on the weighted channel specific sampling sequence, non-linear compression, and parameters of spatial channel interaction.
In accordance with other related embodiments, the electrodes can stimulate the acoustic nerve. The weighting factor can be derived by rectifying the output of each filter, and then determining the maximum amplitude of each half-wave in the rectified signal. The multichannel electrode array can used a monopolar electrode configuration having a remote ground. The pulse amplitudes of the channel specific sampling sequence can be derived by sampling a signal waveform, such as one half the period of a sinusoid between 0 and xcfx80, or one quarter of a sinusoid so that the amplitude distribution is monotonically increasing. Symmetrical biphasic current pulses can be used to sample the waveform. Each channel filter can be a bandpass filter. The duration and number of pulses in the channel specific sampling sequence can then be derived from the center frequency of the channel""s bandpass filter. For example, the duration of the channel specific sampling sequence can be one half of the period of the bandpass filter""s center frequency. The parameters of spatial channel interaction can be based on a single electrode model having exponential decays of the potentials at both sides of the electrode, the sign-correlated pulses having amplitudes determined by using properties of a tri-diagonal matrix. The multichannel electrode array can be in a cochlear implant, whereby the weighting factor is transmitted to the cochlear implant. Start and stop bits, and addresses associated with an electrode can also be transmitted to the cochlear implant.
In accordance with another embodiment of the invention, electrodes are simultaneously activated in a multichannel electrode array using channel specific sampling sequences. Sign-correlated pulses are used. The amplitudes of the sign-correlated pulses are calculated by taking into account parameters of spatial channel interaction. In calculating the amplitudes of the sign-correlated pulses a single electrode model having exponential decays of the potentials at both sides of the electrode can be used. The amplitudes of the sign-correlated pulses can be calculated using properties of a tri-diagonal matrix.
In accordance with another embodiment of the invention, channel specific sampling sequence having a pulse descriptive characterization are defined. The channel specific sampling sequence is used to activate electrodes in a multichannel electrode array, each filter in a bank of filters associated with a channel having an electrode. Pulse amplitudes of the channel sampling sequence are derived by sampling a signal waveform. The duration and number of pulses of the channel specific sampling sequence are derived from a frequency associated with the channel""s filter.
In accordance with other related embodiments, the sampling is of a half period of a sinusoid between 0 and xcfx80. The sampling can also be of a quarter period of a sinusoid between 0 and xcfx80/2, so that pulse amplitude distribution monotonically increases. The sampling can use biphasic current pulses. Each filter can be a bandpass filter. The duration and number of pulses in the channel specific sampling sequence can be derived from the center frequency of the channel""s bandpass filter. The duration of the channel specific sampling sequence can be one half of the period of the bandpass filter""s center frequency.
In another embodiment of the invention, a weighting factor for a channel specific sampling sequence is derived, the channel specific sampling sequence being used to activate electrodes in a multichannel electrode array, each filter in a bank of filters associated with a channel having an electrode. The output of each filter is rectified, creating a half-wave rectified signal. The maximum amplitude of each half-wave in the half-wave rectified signal is then determined.